On a physiological level an emotional hijacking occurs when

Physiology refers to the body function, so the physiological level means: what is happening in the bode when this occurs.

During the emotional hijacking, the emotion that is hijacking the person - beign so strong that it overtakes the whole cognition - is usually fear and the physiological symptoms of fear are sweating, faster heartbeat and high adrenaline - that's also what is typically accompanying emotional hijacking